This invention relates to a process for the production of a liquid hydrocarbon oil or dimethyl ether from a lower hydrocarbon gas containing carbon dioxide.
It is well known to convert a lower hydrocarbon gas (HC gas) to a synthesis gas containing CO (carbon monoxide) and H2 (hydrogen) by reforming reaction thereof with H2O (steam or water) in the presence of CO2 (carbon dioxide). It is also known to produce a liquid hydrocarbon oil (HC oil) having 5 or more carbon atoms suitable for use as a fuel oil by Fischer-Tropsch synthesis (FT synthesis) from the synthesis gas and to produce dimethyl ether from the synthesis gas.
U.S. Pat. No. 4,640,766 discloses reforming in the presence of a Ni catalyst. This process has a problem of carbon deposition on the catalyst, which causes catalytic poisoning.
U.S. Pat. No. 5,621,155 and U.S. Pat. No. 5,620,670 use a Fe catalyst having a high CO shift reaction activity in FT synthesis. About a half of CO in the synthesis gas is lost in the form of CO2 and, hence, the carbon conversion efficiency is at most 50%. The disclosed process uses a Ni catalyst in reforming of HC gas with CO2 and, thus, has a problem of carbon deposition on the catalyst.
Industrial actually employed reforming processes are performed at 600-1,000xc2x0 C. with a steam ratio [H2O]/[C] (ratio of steam to carbon of raw material HC feed) of 2-5. While a lower steam ratio is desired from the standpoint of energy saving, carbon deposition on the catalyst significantly occurs as the steam ratio becomes lower than 2. A higher steam ratio is needed as CO2 concentration in the feed gas increases. This problem is encountered in the above-described conventional processes.
EP-A-0974551 discloses a process for producing a synthesis gas by reacting a hydrocarbon with H2O and/or CO2 using a catalyst having a specific surface area of 25 m2/g or less and comprising a magnesium oxide-containing carrier and Rh and/or Ru supported on the carrier in an amount of 0.0005-0.1 mole %, in terms of elemental metal, based on the carrier. This process is promising because of freedom from the problem of carbon deposition on the catalyst.
In accordance with one aspect of the present invention, there is provided a process for the production of a liquid hydrocarbon oil from a lower hydrocarbon gas and carbon dioxide, comprising the steps of:
(a) mixing a gas feed, containing a lower hydrocarbon having 1-4 carbon atoms and 10-50 mole % of CO2 based on a total mole of the CO2 and the lower hydrocarbon, with H2O to obtain a mixed gas having contents of the CO2, H2O and lower hydrocarbon satisfying the following condition:
0.5xe2x89xa6([CO2]+[H2O])/[C]xe2x89xa62.5 
wherein [CO2] represents the moles of the CO2, [H2O] represents the moles of the H2O and [C] represents the moles of carbon of the lower hydrocarbon;
(b) contacting said mixed gas with a catalyst at a temperature of 600-1,000xc2x0 C. and a pressure of 10-75 atm to produce a synthesis gas with a carbon conversion efficiency Cf of at least 50% and a synthesis gas production efficiency Yf of at least 80%,
said synthesis gas production efficiency Yf being represented by the following formula:
Yf={[CO]+[H2])/([C]+[CO2]+[H2])}xc3x97100% 
wherein [CO] represents the moles of CO in said synthesis gas, [H2] represents the moles of H2 in said synthesis gas, and [CO2], [H2O] and [C] are as defined previously,
said carbon conversion efficiency Cf being represented by the following formula:
Cf={[CO]/([C]+[CO2])}xc3x97100% 
wherein [CO], [CO2] and [C] are as defined previously,
said synthesis gas having a molar ratio of hydrogen to carbon monoxide of 1.5-2.5,
said catalyst having a specific surface area of 5 m2/g or less and comprising a magnesium oxide-containing carrier and at least one catalytic metal selected from the group consisting of rhodium and ruthenium and supported on said carrier in an amount of 10-5,000 ppm, in terms of elemental metal, based on the weight of said carrier;
(c) reacting said synthesis gas in the presence of a Fischer-Tropsch catalyst having a low CO shift reaction activity to obtain a product containing a liquid hydrocarbon oil; and
(d) separating said liquid hydrocarbon oil from said product.
In another aspect, the present invention provides a process for the production of dimethyl ether from a lower hydrocarbon gas and carbon dioxide, comprising the steps of:
(a) mixing a gas feed, containing a lower hydrocarbon having 1-4 carbon atoms and 30-70 mole % of CO2 based on a total mole of the CO2 and the lower hydrocarbon, with H2O to obtain a mixed gas having contents of the CO2, H2O and lower hydrocarbon satisfying the following condition:
0.5xe2x89xa6([CO2]+[H2O ])/[C]xe2x89xa62.5 
wherein [CO2] represents the moles of the CO2, [H2O] represents the moles of the H2O and [C] represents the moles of carbon of the lower hydrocarbon;
(b) contacting said mixed gas with a catalyst at a temperature of 600-1,000xc2x0 C. and a pressure of 10-75 atm to produce a synthesis gas with a synthesis gas production efficiency Yf of at least 80% and a carbon conversion efficiency Cf of at least 50%,
said synthesis gas production efficiency Yf being represented by the following formula:
Yf={[CO]+[H2])/([C]+[CO2]+[H2O])}xc3x97100% 
wherein [CO] represents the moles of CO in said synthesis gas, [H2] represents the moles of H2 in said synthesis gas, and [CO2], [H2O] and [C] are as defined previously,
said carbon conversion efficiency Cf being represented by the following formula:
Cf={[CO]/([C]+[CO2])}xc3x97100% 
wherein [CO], [CO2] and [C] are as defined previously,
said synthesis gas having a molar ratio of hydrogen to carbon monoxide of 0.5-1.5,
said catalyst having a specific surface area of 5 m2/g or less and comprising a magnesium oxide-containing carrier and at least one catalytic metal selected from the group consisting of rhodium and ruthenium and supported on said carrier in an amount of 10-5,000 ppm, in terms of elemental metal, based on the weight of said carrier;
(c) reacting said synthesis gas in the presence of one or more catalysts having activities of methanol synthesis, methanol dehydration and CO shift reaction to obtain a product containing dimethyl ether; and
(d) separating said dimethyl ether from said product.
It is an object of the present invention to provide a process which can produce HC oil on an industrial scale from HC gas by synthesis gas production and succeeding FT synthesis using a minimized size of the synthesis gas production reactor while utilizing not only HC gas but also CO2 as a carbon source of the HC oil.
It is also an object of the present invention to provide a process which can produce dimethyl ether on an industrial scale from HC gas by synthesis gas production and succeeding dimethyl ether synthesis using a minimized size of the synthesis gas production reactor while utilizing not only HC gas but also CO2 as a carbon source of the HC oil.
Other objects, features and advantages of the present invention will become apparent from the detailed description of the preferred embodiments of the invention to follow.